1. Field of the Invention
The present invention generally relates to non-linear optical phase conjugation, and more specifically to an apparatus for combining an array of parallel light beams for phase conjugation and amplification in a master oscillator-power amplifier configuration.
1. Description of the Related Art
A phase conjugate master oscillator-power amplifier (PC MOPA) includes a laser for producing a coherent, high quality light beam which is split into an array of parallel input beams. A power amplifier which may include a laser diode amplifier array amplifies the input beams, which are then directed into a phase conjugate mirror. The beams are phase conjugated and reflected back through the power amplifier and then directed out of the apparatus. The resulting output beam is amplified twice by the power amplifier, and is very clean since no aberrations remain after the two passes through the amplifier. The phase conjugate mirror has the property such that it reverses the sign of the phase of any wave reflected from it. Thus, any phase aberrations picked up by the beams on their first pass through the amplifier exactly cancel out after the second pass. In particular, any phase errors produced by optical path length differences between amplifier elements are compensated for by the conjugation process.
A problem has existed in prior art PC MOPA systems with one- or two-dimensional arrays of amplifiers, and a phase conjugate mirror including crystalline barium titanate (BaTiO.sub.3) whose operation is sensitive to spot size. The parallel light beams must be imaged into a single overlapped spot in the phase conjugator which is invariant with the aberration level of the amplifier. This has been accomplished in the past using conventional optical elements such a wedges and mirrors to process each light beam individually. This expedient is reasonable for small numbers of light beams, but is beyond the limits of practicality for hundreds or thousands of light beams.
For a laser diode PC MOPA to function properly, the aberrations picked up by the input beams on the first pas through the amplifiers must be phase conjugated so that they will be removed on the second pass. Effective phase conjugation can occur only if all the aberration information in the beams is conveyed to the phase conjugator in a useful form. This means that the optical system must collect all of the aberrated beams, and that these beams must be presented to the phase conjugator with the proper size, shape, etc. to enable the conjugator to function. Phase conjugation using barium titanate at aluminum gallium arsenide (AlGaAs) laser diode wavelengths on the order of 830 nm is particularly critical, since if the beam diameter inside the phase conjugator crystal differs by more than .+-.30% from 0.75 mm, either the phase conjugation process will not start, or poor reflectivity will result.
FIG. 1 illustrates a light beam propagating from left to right as viewed in the drawing, and being emitted from a power amplifier 10 after a first pass therethrough. At low amplifier drive current, the output is nearly diffraction limited (non-aberrated) and has low divergence as indicated by solid lines 12. At higher currents, the divergence increases as indicated by broken lines 14 due to higher aberration levels, typically degrading to five times the diffraction limit or worse. If an associated phase conjugate mirror is to function over a wide range of drive currents without major adjustments to the optical system, a portion of the output beam which does not change in size with aberration level must be used inside the barium titanate crystal. As illustrated in FIG. 1, the only portion of the light beam that does not change is a near field (amplifier exit facet or image thereof) 16, since it is constrained by the amplifier dimensions. Thus, in a PC MOPA that utilizes a single amplifier, proper operation of the phase conjugator may be achieved by using a conventional lens system to image the exit facet of the amplifier into the phase conjugator crystal.
However, the exclusive use of lenses cannot cause the near fields of an array of light beams to come together at a single point since the amplifiers are spatially separated, and this separation will be maintained in the image. As illustrated in FIG. 2, an array of parallel light beams 18 emerging from a plurality of respective amplifiers 10 are imaged through a lens 20 located at twice the focal length f of the lens 20 from the amplifiers 10. Images of the beams 18 overlap at a far field 22 which is located one focal length f from the lens 20. A far field is a point at which the intensity profile reaches a steady state, and no longer changes with a further increase in distance. However, the beam size is not invariant with the aberration levels of the amplifiers 10, and will increase as the amplifier current and thereby the aberration level increase. The images of the beams 18 are focussed at a near field 24, but are spatially separated from each other.
It is possible to achieve near field image overlap by individually deflecting the light beams 18 to a common point 26 in space using respective mirrors or optical wedge 28 as illustrated in FIG. 3. For rectangular arrays, each mirror or wedge can be made to deflect a row or column of beams so that as few as N+M-2 elements are required for an N.times.M array (where N and M are odd). However, for arrays with hundreds or thousands of amplifiers and respective light beams, the number of deflecting elements becomes unmanageable. Even if the elements are all fabricated on the same substrate (e.g. a multifaceted axicon or multi-element binary optic array), the resulting element will be complex and expensive. It will also be necessary to fabricate a new element if even small changes are made to the rest of the system.
Light pipes or optical fibers per se have been used in optical systems which include phase conjugate mirrors. An example is found in a paper entitled "Image transmission and interferometry with multimode fibers using self-pumped phase conjugation", by B. Fischer et al, Applied Physics Letters 46 (2), 15 Jan. 1985, pp. 113-114. This reference describes a double-pass image transmission through a single multimode fiber, using a passive phase conjugate mirror. As an application to interferometry based on phase sensing, the multimode fiber and passive phase conjugate mirror were implemented as one arm of a Michelson interferometer. Due to the unique properties of the self-pumped conjugator, nonuniform distortions caused by modal dispersion in the fiber and other aberrations were canceled out, while uniform phase changes were detected.
The Fischer paper teaches how to use a phase conjugate mirror to cancel aberrations in a single light beam propagating through an optical fiber, and does not suggest a solution to the problem of variation in overlap spot size in a phase conjugate mirror used in a PC MOPA which includes a plurality of light beams and amplifiers.